1. Field of the Invention
The present invention relates to a lens system, and particularly relates to a low-cost, small-size and yet high-performance imaging lens system suitable for use in a camera phone and a compact digital camera.
2. Description of Prior Art
Portable imaging devices, such as camera phones, compact digital cameras and personal digital assistances (PDAs), generally require its lens system to be compact and low cost while maintaining a high level of optical performance. Due to the limitation of available space, imaging lenses for the portable imaging devices have dimensions much smaller than those for common imaging cameras and video cameras. Accordingly, camera phones and compact digital cameras generally employ fixed focal length lenses.
As an image pick-up device, conventional spherical glass lenses have been widely adopted in optical industry field because there is a wide range of materials available for making such lenses and the color aberration thereof can be easily corrected. However, the spherical glass lenses have the disadvantages of being difficulty in correcting spherical aberration and astigmatic aberration when they are used in cases having a smaller F-number and a larger wide angle of field. To improve the above-mentioned drawbacks existing in the conventional spherical glass lenses, aspheric plastic lenses or aspheric molded glass lenses have been used to obtain better image quality.
In the current market, most imaging lens systems for compact imaging devices, such as camera phones, are generally composed of two plastic lens elements, or of one glass lens element and two plastic lens elements. U.S. Pat. No. 6,031,670, entitled “Wide-angle Lens”, discloses an imaging lens system composed of two plastic lens elements, one of which is a negative meniscus lens element on the object side and the other is a positive biconvex lens element on the image side. Both of the two plastic lens elements are formed as aspheric lenses for aberration correction. However, the overall length of such a lens system is still very long, thus preventing the lens system from having an even smaller volume to meet the light and compact design requirements for all electronic products. Such a conventional imaging lens system therefore has limited applications.
U.S. Pat. No. 6,441,971, entitled “Compact Lens with External Aperture Stop”, discloses an imaging lens system composed of one glass lens element and two plastic lens elements. The imaging lens system of the '971 patent, which is suitable for use in compact portable electronic devices, includes, from the object side thereof, an aperture stop, a distal meniscus lens element made of glass, and first and second aspheric lens elements made of plastics. The meniscus lens element has a convex surface facing the object side, and is adapted to reduce lens temperature sensitivity and provide most of the focusing power while the first and second aspheric lens elements mainly provide aberration compensation. However, in this configuration, the sensitivity to the surface eccentricity of the lens surfaces is relatively high. Accordingly, manufacturing allowance must be strictly controlled, thereby adversely affecting mass productivity. Further, the configuration of one glass lens element and two plastic lens elements is still not competitive in cost. Also, the overall length and weight of this imaging lens system become longer and heavier as compared to an imaging lens system having only two lens elements. Accordingly, in order to obtain the small size and portability desired by the market, further improvements on such an imaging lens system are needed.